LFA (lateral flow assay, or lateral flow immunoassay, also called rapid diagnostic test—RDT, or bioassays) technology has found widespread use both in and out of laboratory settings. In a typical assay, a fluid sample from a patient is applied to a test strip. The sample interacts with chemicals on the test strip causing the strip to optically change characteristics. The visual indicator may be observed by a person, for example, using a home pregnancy test. However, more accurate readings can be obtained using an assay reader. Such a reader may, for example, include a sensitive optical sensor that is capable of sensing optical variations more accurately and in a more repeatable manner than a human viewer. One example of a typical assay reader is shown in U.S. Pat. No. 7,297,529, to Polito et al., issued Nov. 20, 2007.
The ability to rapidly identify diseases enables prompt treatment and improves outcomes. This possibility has increased the development and use of rapid point-of-care diagnostic devices or systems that are capable of biomolecular detection in both high-income and resource-limited settings. LFAs are inexpensive, simple, portable and robust, thus making LFAs commonplace in medicine, agriculture, and over-the-counter personal use, such as for pregnancy testing. LFAs are also widely used for a number of infectious diseases, such as malaria, AIDS-associated cryptococcal meningitis, pneumococcal pneumonia, and recently tuberculosis.
Although the analytical performance of some LFAs are comparable to laboratory-based methods, the analytical sensitivity (alternatively called limit of detection) of most LFAs is in the mM to μM range, which is significantly less sensitive than other molecular techniques such as enzyme-linked immmunoassays (ELISAs). As a consequence, LFAs are not particularly useful for early detection in a disease course when there is low level of antigen. Research has focused on developing microfluidics, biobarcodes and enzyme-based assay technologies to obtain higher sensitivity in antigen detection since these techniques may potentially detect in the nM to pM range. However, all of these methods are still in the development stage and have not been demonstrated for adoption in a reliable, cost-effective manner to use in a point-of-care site by an end user.
As is now well known, the optical, thermal and electrical properties of materials change dramatically in the nanoscale. In particular, the enhanced photothermal signature of metal nanoparticles have been utilized for: thermal ablation of malignant tumors, detecting circulating tumor cells, photothermal gene transfection, enhancing the therapeutic efficiency of chemotherapeutics, and for tracking the transport of nanoparticles within cells.